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Abstract
We have established a method to control the polymer aggregation structure of the PDLCs by using a micro-lens structure and irradiating them with uni-directionally diffused UV light. The micro-lens structure on the surface of the substrate produced an uneven illuminance distribution of UV light in the LC–monomer mixture, and the monomer was polymerized along the direction of UV light irradiation at positions where the UV illuminance was high and formed a layered structure. The proposed PDLCs could control the light diffusion distribution by the internal polymer aggregation structure and had a high diffusion efficiency without polarization dependency in the haze value.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
It is important to consider light distribution patterns when designing electronic displays and illumination devices. For example, we can improve the light utilization efficiency of liquid crystal displays (LCDs) by focusing light from a backlight unit toward observers. If we can achieve dynamical control of light distribution, we can improve the functionality of automotive headlights, thus improving safety for drivers. Furthermore, suitable light distribution control for electronic displays not only enables users to protect their privacy by controlling the viewing angle range, but also improves power consumption.
As a technique for controlling light distribution patterns, polymer-dispersed liquid crystals (PDLCs) have been widely used in a variety of applications, such as privacy windows and displays with controllable viewing angles [1–4]. PDLCs can electrically switch the optical transmission state between a scattering state and a transparent state by controlling the mismatch between the refractive indices of the liquid crystal (LC) droplets and a polymer network.
However, it is challenging to control the distribution pattern of diffused light precisely due to the difficulties associated with controlling the polymer aggregation structure of PDLCs. This decreases the light utilization efficiency and limits the applicability of this technique [5–7]. Therefore, it is necessary to establish an optimization method for light diffusion distribution through PDLCs.
To solve this problem, we propose a technique for controlling the internal polymer aggregation structure of PDLCs by irradiating a LC–monomer mixture with uni-directionally diffused ultraviolet (UV) light. We successfully fabricated layered polymer structures inside PDLCs and achieved anisotropic light diffusion as shown in Fig. 1 [8,9]. In this paper, we refer to this type of PDLC as layer-structured PDLC. Our method does not require conventional interference irradiation techniques [10–12] and can achieve high diffusion efficiency without depending on the polarization state of the incident light; therefore, the proposed approach makes it easy to control the distribution patterns of diffused light over a large area.
However, there are problems with the fabricated PDLCs, namely low haze values and narrow diffusion angles. This is due to the low density of polymer layer structures. Thus, it is necessary to clarify the formation mechanism of the layered polymer structure to improve the controllability of the polymer aggregation structure and the light scattering efficiency of the PDLCs.
In this paper, we fabricated micro-lens structure on a substrate surface and investigated the light-focusing effects of these structures on the distribution of polymer aggregation structure in the PDLCs. Thus, we established a method to control the polymer aggregation structure and improve the light scattering efficiency of layer-structured PDLCs.
2. Evaluation of cell thickness effect on the internal polymer aggregation structure of PDLCs fabricated using uni-directionally diffused UV light
We investigated the effect of varying the cell thickness on the internal polymer aggregation structure of the fabricated PDLC using uni-directionally diffused UV light to investigate control of the polymer aggregation structures.
Figure 2 presents the procedure for fabricating the layer-structured PDLCs. This process is based on the our previously reported light-diffusing film composed of alternating polymer layers with different refractive indices [13–16]. We prepared a mixture of nematic LCs (E-7; Merck) and UV-curable monomers (NOA65; Norland Products) at a weight ratio of 3:2. Next, we fabricated an empty cell using glass substrates (thickness: 200 μm) without alignment treatment and the LC–monomer mixture was injected into the cell via capillarity action. Finally, the cell was irradiated with uni-directionally diffused UV light (non-polarized), which we adjusted by combining a collimated UV light source (JATEC Co., Ltd.) and a surface modulated light-diffusing plate (Luminit Co., Ltd.) with a diffusion angle of 60° with respect to the y-axis and 1° with respect to the x-axis as shown in Fig. 3. The irradiation area covered the whole sample with uniform UV illuminance and the light source and LC cell did not move during UV curing process. The center wavelength of the UV light was 365 nm. The amount of integrated UV light was 6.0 J/cm2. The phase separation of polymer network and LCs in PDLCs is considerably affected by the temperature of mixture during UV irradiation process [17–19], therefore we set the process temperature to 45 °C to make a LC-monomer mixture in isotropic phase [20,21].
Figure 4 shows the formation mechanism of the layered polymer structure by the uni-directionally diffused UV light in the X-Z plane shown in Fig. 2. The UV light is diffused only to the direction parallel to the Y-Z plane, therefore the UV light can be considered as a collimated light in the X-Z plane. The layered polymer structure was fabricated using the polymerization-induced phase separation (PIPS) method [22–25] and a mixture of LCs and monomers with different refractive indices is irradiated by the uni-directionally diffused UV light. When the mixture was irradiated with UV light, the monomer polymerized and phase separated at random positions in the upper side of the mixture as shown in Fig. 4(b). This polymer aggregates have different refractive index from LC-monomer mixture and refracted and focused the UV light, thus producing an uneven UV illuminance distribution in the LC-monomer mixture as shown in Fig. 4(c).
Fig. 4 Fabrication mechanism of layered polymer aggregation structure by uni-directionally diffused UV light in the X-Z plane.
The diffused monomer moved into the high UV illuminance region and photo-polymerization of the monomer was promoted along the direction of UV light. The alternating layered polymer structure was formed by separating the polymer from the LCs as shown in Fig. 4(d) [26,27]. The layer-structured PDLC has the optical property of anisotropic light diffusion. The layer structure is considered as stacked phase gratings and the anisotropic light diffusion is due to the nature of multiple diffraction by microstructures with anisotropically distributed refractive indices [28,29].
We observed the internal polymer aggregation structure of PDLCs using a polarizing microscope (BX-50, Olympus Co., Ltd.) under the crossed-Nicols polarizer. The dark and bright region under the crossed-Nicols polarizer indicates the presence of many polymers with isotropic refractive indices and LCs with birefringence. Therefore, we evaluated the formation of the LC-polymer structure based on microscope images.
We measured the distribution of diffused light through the layer-structured PDLCs using an angle-luminance analyzer (Conoscope, Autronic-MELCHERS GmbH) equipped with a collimated white light emitting diode (LED). The white LED was incident normally to the sample. The diffusion angle was defined as the full width at half maximum (FWHM) based on the angle-luminance characteristics of the diffused light, excluding the transmitted straight light as shown in Fig. 5.
The amount of the light diffused (haze value) by the PDLC was measured using a haze meter (HM-150, Murakami Color Research Laboratory Co., Ltd.). The haze value indicates the ratio of the total light transmittance and the diffused light transmittance and is defined by the following equation.
Figure 6 presents the results of the measurements of the internal polymer aggregation structure and the distribution pattern of the light diffused through the layer-structured PDLCs fabricated with cell thicknesses from 10 to 100 μm.Fig. 6 Cell thickness dependence of the polymer aggregation structure and distribution of diffused light.
In the cases of PDLC cells with thicknesses of 10 μm and 50 μm, the internal polymer aggregation structure was a blend of random polymer network and LC droplets as shown in Figs. 6(a) and 6(b). This structure arose due to the polymerization-induced phase separation effect and the incident light diffused isotropically. In contrast, when the cell thickness was increased beyond 70 μm, the internal polymer aggregation structure of the PDLC formed an alternating striped LC-polymer pattern along the direction of the UV light diffusion as shown in Figs. 6(c) and 6(d) and the incident light diffused uni-directionally. Hence, it becomes difficult to control the internal polymer aggregation structures of PDLCs by the UV light distribution when the thickness of the cell is decreased.
We considered that this is because the UV illuminance distribution in the LC-monomer mixture did not vary sufficiently to induce a phase separation along the direction of the incident UV light diffusion in the case of thin PDLCs. The illuminance distribution of thin PDLCs is discussed in section 3.2 by using ray tracing numerical simulation. Increasing the cell thickness increases the driving voltage of the PDLCs. Therefore, in the next section we discuss how to control the UV illuminance distribution by using micro-lens structure to achieve suitably thin PDLCs.
3. Control of UV illuminance distribution using micro lens structure
3.1 Principle of the UV distribution control by micro lens structure
In this section, we propose a technique for controlling the internal polymer aggregation structures of thin PDLCs by using micro-lens structure fabricated on a substrate surface instead of polymers. Figure 7 shows the mechanism for controlling the polymer aggregation structure by using micro-lens structure in the X-Z plane. When uni-directionally diffused UV light is incident on the micro-lens structure, the micro-lens structure refracts and focuses the UV light and produce an uneven UV illuminance distribution in the LC-monomer mixture in the X-Z plane. Accordingly, the monomer is polymerized along the direction of UV light, at positions where the UV illuminance is high, and a layered polymer structure forms in the PDLCs. This enables us to use micro-lens structure to precisely control the polymer aggregation structures.
3.2 Calculation of UV illuminance distribution in LC layer
We calculated the illuminance distribution of UV light in LC layer by using ray tracing simulation software (LightTools; Cybernet) to investigate the light-focusing effects of the micro-lens structure on the surfaces of the substrates. We calculated the distribution of UV light in the X-Z plane shown in Fig. 8. The UV light is diffused only to the direction parallel to the Y-Z plane by the surface modulated light-diffusing plate, therefore the UV light can be considered as a collimated light and incident normally to the LC cell in the X-Z plane.
Fig. 8 Fabrication method of layer-structured PDLCs using a polycarbonate substrate with micro-lens structure.
Figure 9 presents the conditions used in the calculation of the illuminance distribution in the LC-monomer layer. The diameter, height, and pitch of the lens structure were set to 10, 0.7, and 60 μm, respectively. The thicknesses of the substrate and LC–monomer layer were set to 80 and 10 μm, respectively, with refractive indices of 1.59 and 1.57.
Figure 10 shows the illuminance distribution of UV light in the A-A’ plane of the LC–monomer layer, both with and without micro-lens structure. In the case without micro-lens structure, the illuminance distribution of UV light was uniform in the A-A’ plane as shown in Figs. 10(a) and 10(b), while the UV illuminance was distributed non-uniformly, increasing at the positions where the light was refracted by the micro-lens structure as shown in Figs. 10(c) and 10(d). Hence, we confirmed that we can control the illuminance distribution of UV light in LC-monomer layers by using micro-lens structure, even in thin PDLCs.
We also calculated the illuminance distribution of UV light in the plane located close to the micro lens (B-B’ plane) assuming the case of thin PDLCs discussed in section 2. The distance between micro lens and B-B’ plane was 10 μm. As shown in Fig. 11, the variation of illuminance distribution of UV light was small in the B-B’ plane because of the small focusing effect of micro lens. As a result, we confirmed that distribution control of UV illuminance by the polymer is difficult in the case of thin PDLCs.
Fig. 11 Calculation result of the illuminance distribution of UV light in the plane located close to the micro lens.
3.3 Evaluation of internal polymer aggregation structures of PDLCs using a polycarbonate substrate with micro-lens structure
We fabricated layer-structured PDLCs using micro-lens structure on a polycarbonate substrate and observed the internal polymer structure of the PDLCs to experimentally confirm the light focusing effects of the micro-lens structure.
The fabrication procedure of the layer-structured PDLCs is as follows. We prepared a mixture composed of nematic LCs (E-7; Merck) and UV-curable monomers (NOA65; Norland Products) at a weight ratio of 3:2. Next, we fabricated an empty cell using polycarbonate substrates (thickness: 80 μm) with micro-lens structure (diameter: 10 μm; height: 0.7 μm), as shown in Fig. 12. The cell gap was 10 μm. The LC–monomer mixture was injected into the cell via capillarity action. Finally, the cell was irradiated with uni-directionally diffused UV light at a diffusion angle of 60° along the y-axis direction and 1° along the x-axis direction. The integrated amount of UV light was 6.0 J/cm2, and the process temperature was 45°C.
Figure 13 presents microscope images of the PDLCs fabricated using polycarbonate substrates and with micro-lens structure in the X-Y plane. Figure 14 presents the results of the measurements of PDLCs using glass substrates for comparison. In the case of PDLCs using polycarbonate substrates, a layered polymer structure formed along the direction of the UV light diffusion as shown in Fig. 13(a), while the PDLCs using glass substrate had an isotropically distributed polymer aggregation structure, as shown in Fig. 14. Furthermore, we observed that the positions of the micro-lens structure corresponded to the centers of these polymer aggregation structures when we focused on the polycarbonate surface as shown in Fig. 13(b).
Fig. 13 Microscopy images of PDLCs fabricated using a polycarbonate substrate with micro-lens structure in the X-Y plane focused at (a) LC–polymer layer and (b) substrate surface.
We considered that the monomer was selectively polymerized underneath the micro-lens structure in locations with strong UV illuminance. As a result, we clarified that the micro-lens structure focused the UV light and produced an uneven UV illuminance distribution in the LC layer. Accordingly, the monomer was polymerized along the direction of UV light irradiation, at positions where the UV illuminance was high. Thus, we can control the internal polymer aggregation structure of thin PDLCs using micro-lens structure.
We demonstrated the electrical switching of the light distribution pattern of the layer-structured PDLCs. The cell thickness was 10 μm and a monochromatic laser light with a wavelength of 650 nm was incident normally on the PDLCs. Figure 15 presents photographs, isoluminance contours and distribution pattern of diffused light in voltage off- and on-states. The applied voltage was 50 volts in on-state. The diffusion angle was 12.3 deg. and 2.2 deg. and the haze value was 57.9% and 8.9%, respectively. Figure 16 shows the voltage-haze properties of the layer-structured PDLCs. These figures confirm that the layer-structured PDLCs can electrically switch the light distribution pattern between anisotropic light diffusion and light transmission. However, we obtained a haze value of 60% in the voltage-off state, which is relatively low for optical devices. This was due to the low density of polymer layer structures shown in Fig. 13(a).
Fig. 15 Isoluminance contours and distribution pattern of light diffusion by layer-structured PDLCs in (a) off-state and (b) on-states (50 volts).
3. 4 Fabrication of lens structure by photolithography
We fabricated dense lens structure using photolithography techniques. The fabrication procedure and conditions are shown in Fig. 17 and Table 1, respectively. We spin-coated the negative type photoresist PR-200 (Osaka Organic Chemical Industry Ltd.) on the glass substrate and baked it at 90°C for 10 minutes as shown in Fig. 17(a). Next, we irradiated the sample with UV light though photomasks and cured the photoresist as shown in Fig. 17(b). The photomasks have periodic or random patterns of circles with apertures of 5 μm as shown in Fig. 18. The center wavelength of the UV light was 365 nm, and the integrated amount of UV light was 2.4 J/cm2. We then removed the uncured photoresist using developer containing 2.38% tetra methyl ammonium hydroxide (TMAH) to fabricate lens structure on the glass substrate as shown in Figs. 17(c) and 17(d).
Table 1. Fabrication conditions of micro-lens structure.
Figure 19 presents the measurement results of the surface structures of the micro-lens. We confirmed that the micro-lenses were periodically or randomly formed according to the photomask pattern. The number density of lens structure was 10,000/mm2 which is higher than the polycarbonate substrate used in the previous section (1,500/mm2), the diameter was 7.1 μm, and the height was 1.4 μm.
Fig. 19 Measurement results of fabricated micro-lens structure of (a) periodic and (b) random types.
3. 5 Fabrication of layer-structured PDLCs by using dense micro-lens structure
We fabricated layer-structured PDLCs by using dense micro-lens structure. We placed the glass substrate with micro-lens structure on the LC cell and irradiated the sample with uni-directionally diffused UV light, as shown in Fig. 20.
Fig. 20 Fabrication method of layer-structured PDLCs using an additional substrate with micro-lens structure.
Figure 21 shows the microscopy images of the layer-structured PDLCs fabricated using periodically or randomly arranged micro-lens structure. Figures 21(a) and 21(b) show that the layer-structured PDLCs had a periodic polymer layer structure with a layer pitch of 10 μm, and the incident light was diffracted in one direction, perpendicularly to the layer structure. In contrast, random polymer layer structures formed on the entirety of the LC cell with random micro-lens structures and the diffused light was distributed continuously with respect to the angle as shown in Figs. 21(c) and 21(d). Hence, we confirmed that the high-density polymer layer structure can form an entire LC cell by using micro-lens structure and uni-directionally diffused UV light. The distribution pattern of the diffused light can be controlled by adjusting the internal polymer aggregation structure of the PDLCs.
Fig. 21 Comparison of polymer aggregation structure and distribution of diffused light between (a) PDLC with periodic polymer layer structure and (b) PDLC with random polymer layer structure.
Finally, we compared the internal polymer aggregation structure and light distribution pattern of diffused light. As can be seen from Fig. 22, the density of the polymer layer structure was increased by the micro-lens structure, the diffusion angle increased from 12.7° to 20.1°, and the haze value improved, increasing from 57.9% to 79.6%, due to the densely formed polymer layers.
Fig. 22 Comparison of polymer aggregation structure, distribution of diffused light and haze value between (a) PDLC with sparse layer structure and (b) PDLC with dense layer structure.
4. Conclusion
To control the distribution of optical diffusion through PDLCs, we proposed a layer-structured PDLC with layered polymer structures. These PDLCs are fabricated by irradiation with uni-directionally diffused UV light. We also investigated the light-focusing effects of micro-lens structure on the distribution of polymer aggregation structure in PDLCs and the light diffusion distribution of the proposed PDLCs.
We confirmed that the micro-lens structure on the surface of the substrate fabricated by photolithography produced an uneven illuminance distribution of UV light in the LC–monomer mixture, and that the monomer was polymerized along the direction of UV light irradiation at positions where the UV illuminance was high. We proposed a method to fabricate dense polymer layers inside the PDLCs by using lens structure and irradiating them with uni-directionally diffused UV light. We successfully established a method to control the polymer aggregation structure of the PDLC and improve the haze value by forming dense polymer layers inside PDLCs.
The layer-structured PDLCs had a high diffusion efficiency without polarization dependency in the haze value. We can electrically switch the light distribution pattern between anisotropic light diffusion and light transmission. Based on these results, we believe that layer-structured PDLCs present a promising technology for future functional displays and illumination devices, and further improvement of the polymer aggregation structure control will enable us to achieve precise control of light diffusion distribution and increase the light utilization efficiency of optical devices.
Funding
JSPS KAKENHI (JP16K06289).
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